A spatially uniform flux of X-rays will be attenuated to varying degrees at positions in a plane perpendicular to the flux as the X-rays pass through a patient as a result of the spatial variations in the thickness and composition of the portions of the patient through which the X-rays pass. This spatial variation in the transmission of X-rays through a patient allows an image of the internal structure of the patient to be formed. However, the typical wide range of X-ray intensities in the X-ray flux issuing from the patient tends to limit the useful information that can be gleaned from the visible image produced by the X-rays. Several factors contribute to the loss of information in the visible image and to errors in quantitative measurements: inadequate detector dynamic range resulting in increased system noise in the regions of low transmission; non-uniform quantum statistical fluctuations across the image (suboptimal exit exposure at portions of the image); degradation of image contrast due to limited detector latitude (e.g., X-ray intensities lying in the range of the film shoulder or toe); and severe degradation of contrast in regions of low transmission due to scatter (and veiling glare in the case of image intensification) from adjacent regions of high transmission.
The problem associated with inadequate detector dynamic range can be illustrated by considering the noise present in digital fluoroscopy systems where television camera noise dominates in the dark portions of the image. At smaller patient thicknesses the quantum noise dominates because the video signal is large compared to the camera noise; whereas, in the areas corresponding to the greatest patient thicknesses, the camera noise dominates. If the signal-to-noise ratio of the camera is not adequate to accommodate the useful dynamic range of the image, there will be objectionable noise in the dark regions. A related effect is the incorrect choice of X-ray operating factors caused by bright spots which confuse peak or area-detection devices during test-shot procedures. If X-ray factors are limited to keep bright spots within the range of signals which can be accommodated by the camera, other regions will have insufficient signal and will suffer excessively from system noise.
Where the detector system noise is small, such as where photographic film is used as the detection medium, the quantum statistical noise dominates. Thus, in chest radiography the contrast sensitivity in the regions of the mediastinum and heart is significantly lower than in the lung field where the intensity of the X-ray flux passing through the patient is greater.
The degradation of image contrast due to limited detector latitude is particularly important with photographic film where the range of transmitted X-ray intensities exceeds the linear portion of the film characteristic curve. The problem is especially severe when scatter reduction devices such as scanned slits are employed, since the image dynamic range in the chest increases greatly.
X-rays scattered from highly transmissive areas in the body reduce contrast in adjacent, darker regions. For example, most of the scatter in chest radiography is due to the highly transmissive lung field rather than the denser regions of the chest because of the greater attenuation of the scattered X-ray photons produced in the denser regions. Similar effects cause significant artifacts in digital angiographic studies of the head where intracranial carotid arteries pass over the dense petrous bone. In this dense region, the arteries appear to have decreased iodine content because cross-scatter from adjacent regions affects the logarithmic amplification of the signal which is employed to render differential iodine signals independent of the local transmission values. The presence of scatter and glare within the image intensifier transfers the signal to the wrong portion of the logarithmic response curve.
Errors can be introduced into quantitative measurements because of non-uniform transmission, as in the measurement of injected iodine where (for small iodine thicknesses) the measured thickness is linearly related to actual thickness, but the constant of proportionality may vary by a factor of two or three as a result of X-ray beam hardening and scattered radiation. Because the scatter field is not uniform, it is not possible to subtract the scatter components in a completely uniform fashion when attempting to measure the thicknesses of iodine injected vessels.
With the exception of computed tomography and digital subtraction angiography, image processing following data aquisition has been largely ineffective in improving image quality. If noise is reduced, high frequency information is also reduced, with a corresponding loss of spatial resolution and local contrast. Contrast enhancements such as high-pass filtration or unsharp masking generally enhance high frequency noise.
Several techniques have been attempted to improve the radiographic image quality. The present invention pertains to techniques which employ an attenuating filter in the path of the X-rays ahead of the patient which compensates for variations in patient thickness and attentuation across the imaging field. Such filters potentially allow the entire imaging field to be placed within the linear region of the film characteristic curve and can allow the use of film with narrower latitude to increase image contrast. Such filters can also reduce spatial variations in the X-ray flux to the image receptor, reducing contrast degradation due to radiation scattered from bright to dark areas, allowing all regions to be imaged with almost maximum signal amplitude to minimize the influence of system and quantum statistical noise. However, presently available compensating filters have not gained wide acceptance in diagnostic radiology due to the difficulty of manufacturing the filters and the need to tailor the filters to the anatomical requirements of each patient and to the X-ray spectrum being used. The construction of relatively detailed filters using prior techniques has proven to be time consuming, so that, with such techniques, a filter could not be constructed and used in a single diagnostic session with a patient.